We have studied the relationship of peptide growth factors to various types of human malignancy in vitro. A series of well-characterized human lung cancer cell lines were tested for the expression of growth factors receptors and the production of soft agar growth factors. Small cell and non-small cell lung cancer cells were found to differ in growth factor receptor expression but both types of lung cancer cells were capable of producing soft agar growth factors. A human mesothelioma cell line was studied in detail and found to produce an EGF-related soft agar growth factor similar to the previously described transforming growth factor (TGF). In addition, since these growth factors are acid stable and of low molecular weight, the urine of cancer patients was tested and found to contain soft agar growth factor activity. Preliminary results indicate the presence of unique molecular species of this activity not found in normal controls. The potential clinical applicability of these observations is being explored.